Investigations on optical and photo-thermal conversion characteristics of BN-EG and BN/CB-EG hybrid nanofluids for applications in direct absorption solar collectors

Abstract

Direct absorption using a nanofluid is an efficient way of harnessing solar energy. Present works aim to investigate hexagonal boron nitride-ethylene glycol (BN-EG) nanofluids and BN mixed with carbon black (CB) in EG (BN/CB-EG) hybrid nanofluids for applications in direct absorption solar collectors (DASCs). Nanofluids were prepared by using the two-step method. Stability and homogeneity were inspected photographically after storage of one month. The optical properties of the nanofluids were measured and computed under varying concentration mixing ratios (CMRs). An absorbed energy fraction of 98.92% was obtained in a 2.0 cm height of 90 ppm/15 ppm BN/CB nanofluid. Photo-thermal conversion performances of the nanofluids were investigated using a small in-house built experimental collector. Temperature-rises in the working fluids were measured under varying durations of exposure to a fixed irradiance, fluid height, and CMRs of the nanoparticles. An enhancement of 34.55% photo-thermal conversion efficiency over that of EG alone was obtained in the 90 ppm/15 ppm BN/CB nanofluid after 1200 s exposure to 437 W/m 2 incident irradiance. Some basic approaches for designing a DASC in an optimized manner are also discussed. It is concluded that BN/CB-EG hybrid nanofluids can be the prospective working fluids in DASCs. • Optical and photo-thermal conversion characteristics were experimentally studied • Absorbed energy fraction of 90 ppm BN-EG is 38.75 times of that of EG • Energy storage efficiency increases by 16.23% using 90 ppm BN-EG nanofluid • Using 20 mm 90 ppm/15 ppm BN/CB-EG nanofluid 98.92% energy is absorbed • Energy storage efficiency increases by 34.55% using 90 ppm/15 ppm BN/CB-EG